Exhaust system utilizing a low-temperature oxidation catalyst

ABSTRACT

An exhaust treatment system is provided having a first exhaust passageway fluidly connected to an engine. A particulate filter and a selective catalytic reduction catalyst are situated within the first exhaust passageway. In addition, the system has a first oxidation catalyst located within the first exhaust passageway upstream of the particulate filter and the selective catalytic reduction catalyst. The system also has a second exhaust passageway fluidly connected to the first exhaust passageway upstream and downstream of the first oxidation catalyst. The exhaust treatment system further has a sensor configured to sense a parameter indicative of an exhaust gas temperature. Furthermore, the system has a controller configured to direct exhaust gas through the first oxidation catalyst when the exhaust gas temperature is below a threshold temperature and direct the exhaust gas through the second exhaust passageway when the exhaust gas temperature is above the threshold temperature.

TECHNICAL FIELD

The present disclosure is directed to an engine exhaust system, and more particularly, to an engine exhaust system utilizing a low-temperature oxidation catalyst.

BACKGROUND

Conventional diesel powered systems for engines, factories, and power plants typically produce emissions that contain a variety of pollutants. These pollutants may include, for example, particulate matter, nitrogen oxides (NOx) such as NO and NO₂, and sulfur compounds. Due to heightened environmental concerns, diesel powered engine exhaust emission standards have become increasingly stringent. The amount of pollutants in the exhaust stream may be regulated depending on the type, size, and/or class of engine.

Several devices and systems have been implemented by engine manufacturers to comply with the regulation of particulate matter and NOx exhausted to the environment. Among these devices and systems are diesel particulate filters (DPF) and selective catalytic reduction (SCR) systems. DPFs are devices consisting of a wire mesh medium or porous ceramic substrate that are designed to trap particulate matter. SCR systems include catalytic devices that facilitate a reaction between ammonia and NOx from the exhaust gas to produce water vapor and nitrogen gas (N₂), thereby removing NOx from the exhaust gas. Occasionally, the DPF and SCR emission control strategies may be combined by supporting an SCR catalyst on a particulate filter substrate.

The performance of DPFs and SCR systems are related to the composition of NOx in the engine's emissions. In particular, the performances of DPFs and SCR systems improve as the percentage of NO₂ in the emissions increases. This is because NO₂ reacts with and oxidizes particulate matter trapped on a DPF. In addition, the chemical reactions in an SCR system reduce NO₂ to N₂ more quickly than NO to N₂. In order to take advantage of the effect increased NO₂ production has on the performances of the DPF and SCR systems, oxidation catalysts are often located upstream of DPF and SCR systems or combined with the DPFs upstream of SCR systems for oxidizing NO in the exhaust to boost NO₂ levels.

Due to the chemical properties of the NOx emissions and oxidation catalysts, the NO₂ synthesizing capability of the above-mentioned configuration may be limited. In particular, as exhaust temperatures increase, it becomes more difficult for the oxidation catalyst to generate NO₂. This is due to the fact that NO₂ generation is an equilibrium reaction, and higher exhaust temperatures generally favor NO production over NO₂ production. In addition, conventional oxidation catalysts require a minimum activation temperature and cannot function at lower exhaust temperatures that favor NO₂ production. Thus, the overall amount of NO₂ that can be generated is limited by temperature. Without the boost in NO₂ levels that would otherwise be produced by the oxidation catalyst, the emissions reduction capability of the DPF and SCR system may likewise be limited.

One attempt to improve the NO₂ synthesizing capability of the oxidation catalyst can be found in U.S. Pat. No. 6,973,776 (the '776 patent) issued to van Nieuwstadt on Dec. 13, 2005. The '776 patent discloses an exhaust gas aftertreatment system having an SCR catalyst fluidly connected to an exhaust passage of an internal combustion engine and positioned in series with a DPF. In addition, an oxidation catalyst and an active lean NOx catalyst (ALNC) are positioned upstream of the SCR catalyst and DPF. Fuel is injected into the exhaust upstream of the ALNC, and urea is injected into the exhaust upstream of the SCR catalyst. A controller regulates the injection of fuel and urea in response to NOx levels detected by NOx sensors located upstream and downstream of the SCR catalyst. Due to its small size, the ALNC can warm up more quickly than the oxidation catalyst and can begin generating NO₂ at lower exhaust gas temperatures. In addition, the chemical reaction that occurs inside the ALNC is exothermic and can be used to warm up the oxidation catalyst thereby permitting the oxidation catalyst to also function at lower exhaust temperatures than would otherwise be possible.

Although the system in the '776 patent may boost the generation of NO₂ at low exhaust temperatures, utilizing an ALNC requires additional components and processes that may add to the complexity and cost of the system. In particular, additional sensors, valves, storage tanks, piping, and computing power are required for the adequate injection of hydrocarbons used by the ALNC. The additional components may increase the likelihood of system failure due to component malfunction, as well as increasing the cost of the system. Also, the complex calculations performed by the controller for injecting the proper quantity of fuel may require greater computing power, thereby increasing the cost of the system.

The disclosed system is directed to overcoming one or more of the problems set forth above.

SUMMARY OF THE INVENTION

In one aspect, the present disclosure is directed toward an exhaust treatment system that includes a first exhaust passageway fluidly connected to an engine. A particulate filter, a selective catalytic reduction catalyst, and a sensor are situated within the first exhaust passageway. In addition, the system includes a first oxidation catalyst located within the first exhaust passageway upstream of the particulate filter and the selective catalytic reduction catalyst. The system further includes a second exhaust passageway fluidly connected to the first exhaust passageway upstream and downstream of the first oxidation catalyst. The sensor is configured to sense a parameter indicative of an exhaust gas temperature. Furthermore, the system includes a controller configured to direct exhaust gas through the first oxidation catalyst when the exhaust gas temperature is below a threshold temperature, and direct the exhaust gas through the second exhaust passageway when the exhaust gas temperature is above the threshold temperature.

Consistent with another aspect of the disclosure, a method is provided for generating NO₂. The method includes sensing a temperature of an exhaust gas, and oxidizing the exhaust gas at a first location only when the exhaust gas temperature is below a threshold temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic illustration of a power system according to an exemplary disclosed embodiment of the present disclosure;

FIG. 2 is a diagrammatic illustration of a power system according to another exemplary disclosed embodiment of the present disclosure;

FIG. 3 is a flow chart depicting an exemplary method of operating the after-treatment exhaust system of FIG. 1; and

FIG. 4 is a flow chart depicting an exemplary method of operating the after-treatment exhaust system of FIG. 2.

DETAILED DESCRIPTION

FIG. 1 illustrates an exemplary power system 10 having an engine 12 that combusts a mixture of air and fuel to generate a mechanical output and a flow of exhaust. For the purposes of this disclosure, engine 12 is depicted and described as a four-stroke diesel engine. One skilled in the art will recognize, however, that engine 12 may be any other type of internal combustion engine such as, for example, a gasoline or a gaseous fuel-powered engine. Engine 12 may include an engine block 14 defining a plurality of cylinders 16, an air intake manifold 18 fluidly connecting cylinders 16 to an air intake passageway 20, and an exhaust manifold 22 fluidly connecting cylinders 16 to an exhaust passageway 24. A piston (not shown) may be slidably disposed within each cylinder 16 to reciprocate between a top-dead-center position and a bottom-dead-center position, and a cylinder head (not shown) may be associated with each cylinder 16.

Cylinder 16, the piston, and the cylinder head may form a combustion chamber 26 fluidly connected to air intake manifold 18 and exhaust manifold 22 via fluid passageways 28 and 30, respectively. In the illustrated embodiment, engine 12 includes six such combustion chambers 26. However, it is contemplated that engine 12 may include a greater or lesser number of combustion chambers 26 and that combustion chambers 26 may be disposed in an “in-line” configuration, a “V”0 configuration, or in any other suitable configuration.

Power system 10 may also include an exhaust treatment system 32 for removing and/or reducing the amount of pollutants in the exhaust produced by engine 12 and released into the atmosphere. Exhaust treatment system 32 may include a bypass 34, a pair of valves 36 and 38, a low temperature oxidation catalyst 40, a combined particulate filter and selective catalytic reduction catalyst (DPF/SCR) 42, a sensor 44 for sensing a temperature of the exhaust gas, and a controller 46 for regulating exhaust flow through DPF/SCR 42 in response to the sensed temperature. DPF/SCR 42 may be fluidly connected to low temperature oxidation catalyst 40 via a fluid passageway 48 and may direct treated exhaust gas into the atmosphere via a fluid passageway 50. Bypass 34 may direct exhaust gas around low temperature oxidation catalyst 40 and may be fluidly connected to exhaust passageway 24 and fluid passageway 48. Valve 36 may be disposed within bypass 34, while valve 38 may be disposed within exhaust passageway 24 downstream of an inlet of bypass 34 and upstream of low temperature oxidation catalyst 40. Valves 36 and 38 may be any type of valve such as, for example, a butterfly valve, a diaphragm valve, a gate valve, a ball valve, a globe valve, or any other valve known in the art. Additionally, valves 36 and 38 may be solenoid-actuated, hydraulically-actuated, pneumatically-actuated or actuated in any other manner to selectively restrict the flow of exhaust through bypass 34 and low temperature oxidation catalyst 40. It is contemplated that exhaust treatment system 32 may be located downstream of any turbochargers present (if any) and/or additional emission control devices disposed within or fluidly connected to exhaust passageway 24 (not shown) such as, for example, exhaust gas recirculation components.

Low temperature oxidation catalyst 40 may include one or more substrates coated with a material that catalyzes a chemical reaction to reduce pollution. For example, low temperature oxidation catalyst 40 may oxidize NO and CO constituents into CO₂ and NO₂, which may be more susceptible to catalytic treatment. In addition, low temperature oxidation catalyst 40 may have a light-off temperature as low as approximately 150 degrees Celsius. The light-off temperature may be the minimum temperature at which the NO₂ generating reaction may occur.

The substrates and catalyst materials of low temperature oxidation catalyst 40 may include alkali metals, alkaline-earth metals, rare-earth metals, precious metals or any combination of elements that may initiate an NO₂-generating reaction at temperatures of approximately 150 degrees Celsius and above. In particular, it is contemplated that low temperature oxidation catalyst 40 may include a combination of elements such as, for example, a platinum catalyst material supported on a Cerium Oxide substrate or a gold catalyst material supported on an iron or titanium substrate, if desired.

DPF/SCR 42 may include a selective catalytic reduction catalyst supported on a particulate trapping substrate. The particulate trapping substrate may be any type of after-treatment device configured to remove one or more types of particulate matter, such as soot and/or ash, from the exhaust flow of engine 12. The integral catalytic material and particulate trapping substrate may trap the particulate matter and remove NOx from the exhaust as the exhaust flows through DPF/SCR 42. In particular, the catalyst material may be packaged with, coated on, or otherwise associated with the substrate. In some embodiments, the substrate may, itself, be a catalytic material. It is contemplated that a separate particulate filter and selective catalytic reduction device may alternatively be included, if desired. It is further contemplated that exhaust treatment system 32 may include multiple combined DPF/SCR devices, multiple separate PDFs and SCR devices, or any combination thereof, if desired.

Sensor 44 may include any type of temperature sensing device known in the art. For example, sensor 44 may include a surface-type temperature sensing device that measures a wall temperature of exhaust passageway 24. Alternatively, sensor 44 may include a gas-type temperature sensing device that directly measures the temperature of the exhaust gas within exhaust passageway 24. Upon measuring the temperature of the exhaust gas, sensor 44 may generate an exhaust gas temperature signal and send this signal to controller 46 via a communication line 52, as is known in the art. This temperature signal may be sent continuously, on a periodic basis, or only when prompted to do so by controller 46, if desired.

Controller 46 may include one or more microprocessors, a memory, a data storage device, a communication hub, and/or other components known in the art and may be associated only with exhaust treatment system 32. However, it is contemplated that controller 46 may be integrated within a general control system capable of controlling additional functions of power system 10, e.g., selective control of engine 12, and/or additional systems operatively associated with power system 10, e.g., selective control of a transmission system (not shown).

Controller 46 may receive signals from sensor 44 and compare the sensed temperature to a threshold temperature stored in or accessible by controller 46. The threshold temperature may be indicative of an upper temperature limit, beyond which low temperature oxidation catalyst 40 may function suboptimally and/or may even be damaged. The threshold temperature may be, for example, approximately 250 degrees Celsius. When the exhaust temperature is at or below the threshold temperature, controller 46 may actuate valves 36 and 38 to direct most or substantially all of the exhaust gas through low temperature oxidation catalyst 40. When the exhaust temperature is above the threshold temperature, controller 46 may direct most or substantially all of the exhaust gas through bypass 34. It is contemplated that when the exhaust temperature is at or below the threshold temperature, controller 46 may direct the exhaust gas through both bypass 34 and low temperature oxidation catalyst 40, if desired.

FIG. 2 illustrates another exemplary power system 10 having components similar to the previously disclosed embodiment except for valves 36 and 38, which may be proportional type valves. The exhaust treatment system 32 disclosed in FIG. 2 may additionally include a high temperature oxidation catalyst 54 positioned within bypass 34. High temperature oxidation catalyst 54 may be a device with a porous ceramic honeycomb-like or metal mesh structure coated with a material that catalyzes a chemical reaction to reduce pollution. For example, high temperature oxidation catalyst 54 may oxidize NO and CO constituents into CO₂ and NO₂, which may be more susceptible to catalytic treatment. Such a catalyst may include any suitable catalytic material, such as, for example, alkali metals, alkaline-earth metals, rare-earth metals, precious metals or combinations thereof. In addition, high temperature oxidation catalyst 54 may have a light-off temperature at or above approximately 200 degrees Celsius.

Controller 46 may regulate the flow of exhaust gas based on temperature signals received from sensor 44 and compare the sensed temperature to a temperature range stored in or accessible by controller 46. The temperature range may be indicative of an upper temperature limit beyond which, low temperature oxidation catalyst 40 may function suboptimally and/or even be damaged and a lower limit beyond which, high temperature oxidation catalyst 54 may function suboptimally. The lower and upper thresholds of the temperature range may be, for example, approximately 200 and 250 degrees Celsius, respectively. When the exhaust temperature is at or below the lower threshold temperature, controller 46 may actuate valves 36 and 38 to direct most or substantially all of the exhaust gas through low temperature oxidation catalyst 40. When the exhaust temperature is above the upper threshold temperature, controller 46 may direct most or substantially all of the exhaust gas through bypass 34. When the exhaust temperature is within the temperature range, controller 46 may direct the exhaust gas through both low temperature oxidation catalyst 40 and high temperature oxidation catalyst 54 in amounts proportional to the sensed temperature.

FIGS. 3 and 4, which are discussed in the following section, illustrate the operation of exhaust treatment system 32. Specifically, FIGS. 3 and 4 illustrate exemplary methods for generating NO₂ at low and high exhaust gas temperatures.

INDUSTRIAL APPLICABILITY

The disclosed exhaust treatment system may generate NO₂ over a wide range of exhaust gas temperatures, thereby improving the performance of an associated DPF and SCR system. By utilizing an oxidation catalyst having a low light-off temperature, the exhaust treatment system may boost NO₂ levels at cold temperatures. When used in conjunction with a conventional oxidation catalyst that functions optimally at higher temperatures, NO₂ may be generated over a wider range of temperatures increasing the overall NO₂ production and performance of the exhaust treatment system. The operation of exhaust treatment system 32 will now be explained.

FIG. 3 illustrates a flow diagram depicting an exemplary method for generating NO₂ in an embodiment of power system 10 having only low temperature oxidation catalyst 40. The method may begin when controller 46 receives temperature signals from sensor 44 indicative of a temperature of the exhaust gas flowing through exhaust passageway 24 (step 100). Controller 46 may compare the exhaust gas temperature to a threshold temperature stored in its memory to determine whether the exhaust temperature is above or below the threshold temperature (step 102). The threshold temperature may be a temperature at which low temperature oxidation catalyst 40 may function suboptimally and/or may even be damaged. Such a threshold temperature may be, for example, approximately 250 degrees Celsius.

When controller 46 determines that the exhaust temperature is below or equal to the threshold temperature (step 102: No), controller 46 may direct a majority of the exhaust gas through low temperature oxidation catalyst 40 (step 104). This may be accomplished by setting valve 36 to a fully closed position and setting valve 38 to a fully open position. Once valves 36 and 38 have been set to their respective desired positions, step 100 may be repeated (i.e. controller 46 may receive new temperature signals from sensor 44 indicative of a new temperature of the exhaust gas).

However, when controller 46 determines that the exhaust temperature is above the threshold temperature (step 102: Yes), controller 46 may direct most of the exhaust gas through bypass 34 (step 106). This may be accomplished by setting valve 36 to a fully open position and setting valve 38 to a fully closed position. Once valves 36 and 38 have been set to their respective desired positions, step 100 may be repeated (i.e. controller 46 may receive new temperature signals from sensor 44 indicative of a new temperature of the exhaust gas).

FIG. 4 illustrates a flow diagram depicting an exemplary method for generating NO₂ in an embodiment of power system 10 having both low temperature oxidation catalyst 40 and high temperature catalyst 54. The method may begin when controller 46 receives temperature signals from sensor 44 indicative of a temperature of the exhaust gas flowing through exhaust passageway 24 (step 200). Controller 46 may compare the exhaust gas temperature to a first threshold temperature stored in its memory to determine whether the exhaust temperature is below the first threshold temperature (step 202). The first threshold temperature may be a temperature below the light-off temperature of high temperature oxidation catalyst 54. In other words, the first threshold temperature may be a temperature, below which high temperature oxidation catalyst 54 may be unable to adequately generate NO₂. For example, the first threshold temperature may be approximately 200 degrees Celsius.

When controller 46 determines that the exhaust temperature is below the first threshold temperature (step 202: Yes), controller 46 may direct most of the exhaust gas through low temperature oxidation catalyst 40 (step 204). This may be accomplished by setting valve 36 to the fully closed position and setting valve 38 to the fully open position. Once valves 36 and 38 have been set to their respective desired positions, step 200 may be repeated (i.e. controller 46 may receive new temperature signals from sensor 44 indicative of a new temperature of the exhaust gas).

However, when controller 46 determines that the exhaust temperature is at or above the first threshold temperature (step 202: No), controller 46 may compare the exhaust gas temperature to a second threshold temperature stored in its memory to determine whether the exhaust temperature is above the second threshold temperature (step 206). The second threshold temperature may be a temperature, at which low temperature oxidation catalyst 40 may function suboptimally and/or may even be damaged. Such a threshold temperature may be, for example, approximately 250 degrees Celsius.

When controller 46 determines that the exhaust temperature is above the second threshold temperature (step 206: Yes), controller 46 may direct most of the exhaust gas through high temperature oxidation catalyst 54 (step 208). This may be accomplished by setting valve 36 to the fully open position and setting valve 38 to the fully closed position. Once valves 36 and 38 have been set to their respective desired positions, step 200 may be repeated (i.e. controller 46 may receive new temperature signals from sensor 44 indicative of a new temperature of the exhaust gas).

However, when controller 46 determines that the exhaust temperature is at or below the second threshold temperature (step 206: No), controller 46 may direct the exhaust gas through both low temperature oxidation catalyst 40 and high temperature oxidation catalyst 54 (step 210). This may be accomplished by setting valves 36 and 38 to partially or fully open positions. It is contemplated that, the volume of exhaust gas flowing through each oxidation catalyst may be essentially equal or set at a fixed ratio where one oxidation catalyst receives more exhaust gas than the other, if desired. If it is desired that the volume of exhaust gas flowing through each oxidation catalyst is essentially equal, valves 36 and 38 may be set to fully open positions. However, if it is desired that the distribution of exhaust gas be unequal at a fixed ratio, valves 36 and 38 may be set to positions partially restricting the flow of exhaust.

It is further contemplated that the distribution of exhaust gas through each oxidation catalyst may be modified as the exhaust temperature changes. For example, the volume of exhaust gas flowing through high temperature oxidation catalyst 54 may be increased as the exhaust temperature approaches the second temperature threshold. In addition, the volume of exhaust gas flowing through low temperature oxidation catalyst 40 may be increased as the exhaust temperature approaches the first temperature threshold. Once valves 36 and 38 have been set to their respective desired positions, step 200 may be repeated (i.e. controller 46 may receive new temperature signals from sensor 44 indicative of a new temperature of the exhaust gas).

By utilizing an oxidation catalyst having a low light-off temperature, the emissions treating capability of the disclosed exhaust treatment system may be improved. Utilizing the low temperature oxidation catalyst can boost the system's ability to generate NO₂ in cold conditions, which in turn improves the exhaust treatment system's ability to treat engine exhaust. In addition, because the low temperature oxidation catalyst may require minimal infrastructure to properly function, there may be little impact on the complexity and cost of the system. Furthermore, utilizing both low temperature and high temperature oxidation catalysts may allow the system to generate NO₂ over a greater range of temperatures, which may also boost the performance of the after-treatment exhaust system.

It will be apparent to those skilled in the art that various modifications and variations can be made in the disclosed system without departing from the scope of the disclosure. Other embodiments will be apparent to those skilled in the art from consideration of the specification disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope being indicated by the following claims and their equivalents. 

1. An exhaust treatment system, comprising: a first exhaust passageway fluidly connected to an engine; a particulate filter located within the first exhaust passageway; a selective catalytic reduction catalyst situated within the first exhaust passageway; a first oxidation catalyst located within the first exhaust passageway upstream of the particulate filter and the selective catalytic reduction catalyst; a second exhaust passageway fluidly connected to the first exhaust passageway upstream and downstream of the first oxidation catalyst; a sensor configured to sense a parameter indicative of an exhaust gas temperature; and a controller configured to direct exhaust gas through the first oxidation catalyst when the exhaust gas temperature is below a threshold temperature, and direct the exhaust gas through the second exhaust passageway when the exhaust gas temperature is above the threshold temperature.
 2. The exhaust treatment system of claim 1, wherein the threshold temperature is approximately 250 degrees Celsius.
 3. The exhaust treatment system of claim 2, wherein the first oxidation catalyst has a light-off temperature below approximately 200 degrees Celsius.
 4. The exhaust treatment system of claim 3, further including a second oxidation catalyst located within the second exhaust gas passageway and having a light-off temperature above approximately 200 degrees Celsius.
 5. The exhaust treatment system of claim 4, wherein the controller is configured to direct exhaust gas in parallel through both the first and second exhaust passageways when the exhaust gas temperature is within a predetermined temperature range.
 6. The exhaust treatment system of claim 5, wherein the predetermined temperature range is between approximately 200 degrees Celsius and approximately 250 degrees Celsius.
 7. The exhaust treatment system of claim 1, wherein the first oxidation catalyst has a light-off temperature of approximately 150 degrees Celsius.
 8. The exhaust treatment system of claim 1, wherein the first oxidation catalyst has a platinum based catalyst supported on a platform including cerium.
 9. The exhaust treatment system of claim 1, wherein the first oxidation catalyst has a gold based catalyst supported on a titanium or iron platform.
 10. A method for generating NO₂, comprising: sensing a temperature of an exhaust gas; and oxidizing the exhaust gas at a first location only when the exhaust gas temperature is below a threshold temperature.
 11. The method of claim 10, wherein the threshold temperature is approximately 250 degrees Celsius.
 12. The method of claim 10, further including oxidizing the exhaust gas at a second location only when the exhaust gas temperature is above the threshold temperature.
 13. The method of claim 12, wherein the threshold temperature is approximately 200 degrees Celsius.
 14. A power system, comprising: an engine configured to combust a fuel/air mixture and produce a flow of exhaust; a first exhaust passageway fluidly connected to receive the flow of exhaust from the engine; a particulate filter situated within the first exhaust passageway; a selective catalytic reduction catalyst situated within the first exhaust passageway; a first oxidation catalyst located within the first exhaust passageway upstream of the particulate filter and the selective catalytic reduction catalyst; a second exhaust passageway fluidly connected to the first exhaust passageway upstream and downstream of the first oxidation catalyst; a sensor configured to sense a parameter indicative of an exhaust gas temperature; and a controller configured to direct exhaust gas through the first oxidation catalyst when the exhaust gas temperature is below a threshold temperature, and direct the exhaust gas through the second exhaust passageway when the exhaust gas temperature is above the threshold temperature.
 15. The power system of claim 14, wherein the threshold temperature is approximately 250 degrees Celsius.
 16. The power system of claim 15, wherein the first oxidation catalyst has a light-off temperature below approximately 200 degrees Celsius.
 17. The power system of claim 16, further including a second oxidation catalyst located within the second exhaust gas passageway and having a light-off temperature above approximately 200 degrees Celsius.
 18. The power system of claim 17, wherein the controller is configured to direct exhaust gas through both the first and second exhaust passageways in parallel when the exhaust gas temperature is within a predetermined temperature range.
 19. The power system of claim 18, wherein the predetermined temperature range is between approximately 200 degrees Celsius and approximately 250 degrees Celsius.
 20. The power system of claim 14, wherein the first oxidation catalyst has a light-off temperature of approximately 150 degrees Celsius. 